1. Field of the Invention
The present invention relates to an electroluminescent display device, and more particularly, to an organic electroluminescent display device and a method of fabricating the same.
2. Discussion of the Related Art
In general, an organic electroluminescent display device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons and the holes to generate an exciton, and by transiting the exciton from an excited state to a ground state. Since the organic electroluminescent display device does not require an additional light source due to its self-luminescence property, the organic electroluminescent display device has a small size and is light weight, as compared to a liquid crystal display device. The organic electroluminescent display device also has a low power consumption, high brightness, and a short response time. Thus, the organic electroluminescent display device is used in many consumer electronic applications, such as cellular phones, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm PCs. In addition, the organic electroluminescent display device can have reduced manufacturing costs because of its simple manufacturing processes.
Organic electroluminescent display devices may be categorized into passive matrix-type and active matrix-type depending upon the method used to drive the device. Passive matrix-type organic electroluminescent display devices have a simple structure and are fabricated through a simple manufacturing process. However, the passive matrix-type organic electroluminescent display devices have a high power consumption, thereby preventing use in large area displays. Furthermore, in passive matrix organic electroluminescent display devices, the aperture ratio decreases due to the increased number of electrical lines. Thus, the passive matrix-type organic electroluminescent display devices are commonly used as small-sized display devices. Active matrix-type organic electroluminescent display (AMOELD) devices are commonly used as large-sized display devices since they have a high luminous efficacy, and provide high definition images.
FIG. 1 is a cross sectional view of an active matrix-type organic electro-luminescent display (AMOELD) device according to the related art. In FIG. 1, the AMOELD device 10 includes a first substrate 12 and a second substrate 28, which are spaced apart and face each other. The first substrate 12 is transparent and flexible. A plurality of thin film transistors T and a plurality of first electrodes 16 are formed on an inner surface of the first substrate 12, in which each of first electrodes 16 are connected to the respective thin film transistor T. Organic layers 18 are formed on the first electrodes 16 and the thin film transistors T, and a second electrode 20 is formed on the organic layers 18. The organic layers 18 emit light of three colors: red (R), green (G), and blue (B) within a pixel region P, and are generally formed by patterning an organic material that emits one of red, green and blue.
A desiccant 22 is formed on an inner surface of the second substrate 28 to remove any external moisture and air that may permeate into a space between the first and second substrates 12 and 28. The inner surface of the second substrate 28 is patterned to form a groove, and the desiccant 22 is disposed within the groove and is fastened with a tape 25.
A sealant 26 is formed between the first and second substrates 12 and 28 to attach the first and second substrates 12 and 28, and surrounds elements, such as the thin film transistors T, the first electrodes 16, the organic layers 18, and the second electrodes 20. The sealant 26 forms an airtight space to protect the elements from the external moisture and air.
In the above OELD device, the first electrode 16 functions as an anode electrode and is transparent. Thus, this AMOELD device has a bottom emission type, in which light is emitted through the first electrode 16.
FIG. 2 is an equivalent circuit for a pixel of an organic electroluminescent display (OELD) device according to the related art.
As shown in FIG. 2, a gate line 36 is formed along one direction of a substrate 30 and a data line 49 perpendicularly crosses the gate line 36. A switching element TS is formed at a crossing point of the gate line 36 and the data line 49, and a driving element TD is electrically connected to the switching element TS.
Since the driving element TD is a p-type thin film transistor, a storage capacitor CST is disposed between a source electrode 52 of the driving element TD and a gate electrode 34 of the driving element TD, and a drain electrode 54 of the driving element TD is connected to an anode electrode of an organic electroluminescent diode E, which corresponds to a first electrode 16 of FIG. 1. The source electrode 52 of the driving element TD is also connected to a power line 62.
The OELD device having the above structure can be driven as follows.
First, when a gate signal is applied to a gate electrode 32 of the switching element TS, a current signal flowing through the data line 49 is changed into a voltage signal through the switching element TS and is applied to the gate electrode 34 of the driving element TD. Then, the driving element TD turns on, and thus the gray scale is realized by determining levels of the current flowing through the organic electroluminescent diode E.
At this time, because signals stored in the storage capacitor CST maintain the signal of the gate electrode 34 of the driving element TD, the level of the currents flowing through the organic electroluminescent diode E is kept constant until a next signal is applied even if the switching element TS turns off. The switching element TS and the driving element TD may be an amorphous silicon thin film transistor or a polycrystalline silicon thin film transistor. The amorphous silicon thin film transistor is more simply manufactured as compared with the polycrystalline silicon thin film transistor.
The above OELD device is manufactured by attaching a substrate having array elements and organic luminescent diodes with another substrate for encapsulation. Since the yield of the OELD device depends on the yields of the thin film transistor and the organic light-emitting layer, the whole processing yield is largely affected by processes for forming the organic light-emitting layer that occur at a later stage. Thus, even if the array elements are properly well formed, if the organic light-emitting layer to be formed to a thickness of about 1,000 Å is improperly formed due to impurities or other factors, the resulting OELD device is rejected as bad. Accordingly, the yield of the OELD device is reduced because of the impurities and other factors in the organic light-emitting forming processes, thereby resulting in a loss of manufacturing costs and source materials for the thin film transistor.
The bottom emission mode OELD device has a good stability and a certain degree of freedom in its manufacturing processes. However, the bottom emission mode OELD device has a reduced aperture ratio. Thus, the bottom emission mode OELD device is not suited for a high aperture device.
On the other hand, a top emission mode OELD device has a high aperture ratio, and is easy to manufacture. Additionally, the top emission mode OELD device has a long lifetime. However, in the top emission mode OELD device, since a cathode electrode is generally disposed over the organic light-emitting layer, a choice of material with which to make the cathode electrode is limited. Accordingly, the transmittance of light is limited, and a light-emitting efficacy is reduced. Furthermore, in order to improve the light transmittance, the passivation layer should be formed as a thin film, whereby the exterior moisture and air are not fully blocked.